The invention relates to a pressure sensor for determining the pressure of a process medium. The pressure sensor has a ceramic pressure-measuring cell with a diaphragm which is exposed to the process medium, is axially fixed in a metallic housing and generates an electrical signal which can be picked off and corresponds to a deflection of the diaphragm caused by the pressure.
Usually, with pressure sensors of this type the ceramic pressure-measuring cell is pressed against a stop by means of a metallic fixing device in the housing, said fixing device being directly in contact with said pressure-measuring cell. Temperature changes result in undesirable mechanical stresses between the pressure-measuring cell and the fixing device (and also the housing), said stresses originating from different thermal expansion coefficients of the materials used. For example, rising temperatures cause the metal housing and the metallic fixing device, serving to fix the pressure-measuring cell, to expand to a greater extent than the ceramic pressure-measuring cell. Displacements of individual parts relative to one another can likewise occur, said parts changing their relative position with respect to one another. In some cases these displacements have not been fully reversible in the course of a subsequent temperature decrease, and permanent deformations have remained as a result. Such deformations result in an undesirable temperature hysteresis which falsifies the measured pressure value or values.
DE-A 42 34 290 describes a ceramic pressure-measuring cell which is exposed directly to a process medium and is fixed in a metallic housing by means of a metallic fixing device, a so-called fixing sleeve. The subject matter of DE-A 42 34 290 serves to support the ceramic pressure-measuring cell radially and to relieve it to the greatest possible extent of axial stresses due to the fixing in the housing. For this purpose, DE-A 42 34 290 also proposes (inter alia) a ceramic disk (called xe2x80x9cceramic supporting bodyxe2x80x9d therein) which is arranged in the axial direction between the ceramic pressure-measuring cell and the metallic fixing device (called xe2x80x9cfixing sleevexe2x80x9d therein) and whose thermal expansion coefficient corresponds to that of the pressure-measuring cell.
The subject matter of DE-A 42 34 290 has the disadvantage, however, that the ceramic disk mentioned is fixedly joined to the pressure-measuring cell by means of active braze or glass solder. Different heat-induced expansions of the materials that adjoin one another cause, however, as outlined above, undesirable radial stresses between the metallic fixing device and the ceramic disk. Owing to its strong brazed fixture connection to the pressure-measuring cell, the ceramic disk passes the thermally induced radial stresses on to the pressure-measuring cell, which leads to the abovementioned falsifications of the measured pressure values and/or to temperature hysteresis effects.
Therefore, it is an object of the invention to provide a pressure sensor which avoids temperature hysteresis effects caused by stresses between the pressure-measuring cell and the housing generated by heat-induced expansions.
In order to achieve this object, a first variant of the invention consists in a pressure sensor for determining the pressure of a process medium,
having a metallic housing,
which is open to the process medium and
which has a throughhole for inserting a ceramic pressure-measuring cell therein,
which comprises a ceramic substrate and
a ceramic diaphragm, which is fitted thereon and is exposed to the process medium, and also
means, which supply an electrical signal which can be picked off and corresponds to a deflection of the diaphragm caused by the pressure acting thereon, whereby
the diaphragm is supported axially in a resilient manner on a first stop in the housing and
the substrate is supported on a non-metallic intermediate body, which
is retained by a metallic fixing device in the housing, and whereby
the intermediate body is movable radially with respect to the metallic fixing device.
A preferred embodiment of the first variant of the invention provides a pressure sensor in which the metallic fixing device is a metallic ring or a disk which is fixed in a releasable manner in the housing.
In order further to achieve the object, a second variant of the invention consists in a pressure sensor for determining the pressure of a process medium,
having a metallic housing,
which is open to the process medium and
which has a throughhole for inserting a ceramic pressure-measuring cell therein, which comprises
a ceramic substrate and
a ceramic diaphragm, which is fitted thereon and is exposed to the process medium, and also
means, which supply an electrical signal which can be picked off and corresponds to a deflection of the diaphragm caused by the pressure acting thereon, whereby
the diaphragm is supported axially in the housing in a resilient manner on a process connection and
the substrate is supported on a non-metallic intermediate body,
which is retained by a metallic fixing device in the housing, and
the intermediate body is movable radially with respect to the metallic fixing device.
In a preferred embodiment of the first or second variant of the invention, the intermediate body of the pressure sensor consists of a material whose thermal expansion coefficient corresponds to that of the ceramic substrate of the pressure-measuring cell.
In a particular embodiment of this embodiment, the intermediate body is composed of ceramic material.
According to a further preferred embodiment of the first or second variant of the invention, the intermediate body has a centering part, which at least partially encompasses the pressure-measuring cell radially for the purpose of centering the pressure-measuring cell and the intermediate body in the housing.
According to still another preferred embodiment of the first or second variant of the invention, recesses are provided in a first end face of the intermediate body, said first end face resting on the pressure-measuring cell, through which recesses electrically conductive connections are routed. The advantage is that shields against interfering electromagnetic irradiation of an electronic portion and of the pressure-measuring cell are thereby connected conductively to one another in a simple manner.
According to still another preferred embodiment of the first or second variant of the invention, a film made of cold-flowing material is arranged between the intermediate body and the fixing device. This embodiment minimizes the friction between the fixing device and the intermediate body of the pressure sensor, said friction being caused by heat-induced expansions.
A further advantage of the invention is that it not only enables stresses which occur in the radial direction and act on the pressure-measuring cell to be relieved but also permits, in a simple manner, the control of stresses which act on the pressure-measuring cell in the axial direction caused by different heat-induced expansions. By a suitable choice of the axial extent of the intermediate body (height and/or thickness of the intermediate body) taking account of resilient and/or elastic properties of a seal with respect to the process medium, axial loading on the pressure-measuring cell can also be kept as low as possible and mostly constant.